The energy and time efficiency of wireless sensor networks (WSNs) is frequently affected by the low reliability of their links. To mitigate the outage probability, cooperation topologies are used. However, these topologies have particular challenges since the relay consumes energy in assisting a foreign communication, and the successful transmission in each direction is conditioned to the availability of the two segments involved in the communication. To overcome the temporary unavailability of a link, the use of buffers in the relay has been proposed, but energy and time efficiency remain a challenge for basic configurations. We propose two-way buffer-aided relaying strategies that exploit the presence of buffers in the different nodes that participate in the communication, as well as the efficient use of buffer capacity. The proposed strategies make the decision to forward the messages in one of the communication directions or broadcast coded messages based on buffer and channel state information. Firstly, we evaluate the impact of considering the use of buffers in the transmitter nodes. Then, we propose and evaluate the impact of the full and joint use of the entire buffer capacity to assist communication in both directions. Finally, we evaluate the performance of a system that exploits both the use of full and joint buffering and the presence of buffering in the transmitter. The results show that better performance, in terms of outage probability, is obtained by the third strategy; since it allows the buffer capacity to be exploited to a greater extent in the most urgent direction at each moment, as well as to select the start of each transmission to a viable communication opportunity. This represents a notable benefit in terms of energy and time efficiency for WSNs since unnecessary transmission of information is avoided, the number of idle slots decreases, and the amount of information per unit of time and energy increases.

In this paper, we study a buffer-state-based relay selection scheme in a underlay cognitive radio (CR) non-orthogonal multiple access (NOMA) network limited by the interference limit accepted by the primary. To model the state of the relays in relation to packet numbers, it was necessary to implement the Markov chain model. The results of the simulations performed show validity of the proposed scheme. More specifically, in different relay position scenarios analyzed, results show that the best configuration is when the relay is closer to the nodes of the NOMA network. Beyond that, proper selection of the power allocation factor produces a better performance in outage probability. Finally simulation results show that the size of the relay buffer can improve the scheme and achieve full diversity.